Numerous processes have been developed over the years for biological treatment of domestic and industrial wastewater and sludge. These biological treatment processes include both aerobic and anaerobic treatment processes which use biologically active microorganisms (or "biomass") to convert various soluble contaminants, especially organic contaminants, into a form which can be separated from the wastewater. Insoluble organic contaminants are digested by such processes to produce a reduced quantity of a known biomass.
Although such biological treatment processes yield purified water, they also yield a net positive production of biological solids which contribute to the biomass used for the process. Consequently, a portion of the biomass (or sludge) must be periodically removed for further treatment or disposal. There are significant costs associated with further treatment of the biomass produced, and there are numerous regulations and significant costs associated with disposal of the biomass. Thus, in recent years, much attention has been focused on minimizing the amount of biomass produced during the biological treatment process.
In the mesophilic temperature range, about 50.degree. F. to 110.degree. F., the new bacteria generated as a result of the anaerobic treatment process is only approximately 10 to 20 percent of that produced as a result of the aerobic treatment process. Furthermore, because methane from the biogas produced during anaerobic treatment can be burned to provide heat to the system, the energy requirement for the anaerobic process is only about 10 to 20 percent of that for the aerobic process at the same temperature. A substantial drawback to the anaerobic treatment process, however, is that it produces a biogas which consists primarily of methane (CH.sub.4), carbon dioxide (CO.sub.2) and hydrogen sulfide (H.sub.2 S). The H.sub.2 S in the biogas can cause severe odor problems and biological toxicity. Hydrogen Sulfide is an extremely corrosive gas and can be lethal in high concentrations. With the increasingly stringent federal regulations governing this area, the anaerobic treatment process is becoming less and less desirable for many waste treatment applications.
As an alternative, aerobic treatment processes in the thermophilic temperature range, about 115.degree. F. to 170.degree. F., produce only about 10 to 20 percent of the biomass generated from the aerobic process in the mesophilic range. Additionally, the offgas produced from aerobic treatment primarily comprises carbon dioxide (CO.sub.2), with essentially no hydrogen sulfide (H.sub.2 S) production. Thus, aerobic thermophilic processes yield reduced biomass production comparable to mesophilic anaerobic processes, but without the associated noxious and odorous biogas.
Both soluble wastewater constituents and particulate or suspended solid matter, such as waste biological sludges, can be used as a food or fuel source for the thermophilic microorganisms. Solid matter first has to be biochemically hydrolyzed to soluble constituents and transported across the thermophilic microorganism cell wall before it can be used as a food or fuel source by the microorganisms. The food or fuel value of the waste material is best measured as chemical oxygen demand (COD) or volatile solids (VS).
The COD can be measured or calculated. The calculated or theoretical COD represents the stoichiometric amount of oxygen which would be required to chemically oxidize all of the food, fuel, or organic matter in the waste material to carbon dioxide and water. The COD value can be calculated when the composite empirical formula for the waste materials being oxidized is known along with their relative concentrations. Otherwise, the COD can be measured by a standard COD test methodology used to oxidize all the organic matter to carbon dioxide and water, whereby the associated oxygen equivalents are measured. Likewise, the volatile solids content of a waste can be measured by a standard solids test methodology, whereby the solids are burned in a furnace for gravimetric determination of the amount of solids volatilized or lost. Either the COD, VS, or both can be effectively utilized to measure the amount of substrate, food source, fuel, or organic matter available in the waste for utilization by the thermophilic bacteria for growth, energy production, heat production, and cell maintenance.
As noted, the aerobic thermophilic treatment process generates heat and is, therefore, an exothermic process. Thus, if the waste to be treated is sufficiently concentrated with organic compounds to serve as food for the thermophilic bacteria in the reaction processes, the reaction process will be autothermal, i.e., the reaction will supply enough heat to maintain the temperature at the desired level within the thermophilic range. Even if the waste is not a "high strength" waste, the reaction can also be autothermal if the temperature of the waste is sufficiently high.
Although high strength and high temperature wastes can, in theory, yield an autothermal thermophilic process, in practice the prior art methods have been unable to achieve this result in a commercially practicable process. The primary problem arises from the air that is injected into the reactor to provide the oxygen necessary to react with the thermophilic bacteria. The air utilized for such processes is normally compressed air which has been obtained from the ambient air surrounding the treatment facility. This air is normally not 100% humidified and may have a temperature which is relatively low compared to the desired thermophilic temperature of the sludge. Consequently, as the air rises through the liquid in the reaction vessel it will be humidified and heated. As the air exits the reaction vessel, heat is lost from the system and the temperature of the system is lowered. Thus, prior art methods have required addition of heat to the process from an external source, increasing significantly the cost associated with the treatment process.
Heightening the problem is the fact that in prior art processes only a portion of the oxygen available from the injected air will actually be transferred to the thermophilic microorganisms to supply the oxygen demand required for aerobic treatment. Thus, as more air must be passed through the reaction liquid to meet the oxygen demand because of poor oxygen transfer, more heat is lost in the offgas, and the temperature of the system will not support autothermal conditions. By contrast, if too little air is passed through the system, there will not be enough oxygen transferred to the thermophilic microorganisms for aerobic treatment of the waste, and the process will turn anaerobic with all of the associated disadvantages.
Thus, there continues to be a need for a process for aerobic thermophilic treatment of wastewater and sludge. The process should be autothermal and should match the oxygen transfer to the oxygen demand.